In activated sludge-type systems for reducing the COD and BOD content of industrial and municipal liquid wastes, it has long been proposed to utilize oxygen-rich aeration gases in aeration basins in order to increase the rate of biological and chemical reactions, as opposed to, the use of atmospheric air in such aeration basins, The term "biochemical oxygen demand" will hereafter be used to include both the chemical oxygen deman (COD) and the biological oxygen demand (BOD). It has also been taught, for example, in U.S. Pat. Nos. 3,547,815 and 3,725,258 that the provision of walls or partitions which divide the aeration chamber into a plurality of physically separated liquid stages can be beneficial in progressively reducing the biochemical oxygen demand of the mixed liquor in a basin of minimum length by forcing the mixed liquor through successive liquid stages from the inlet end to the outlet end of the basin without backflow of the mixed liquor from a stage of lesser biochemical oxygen demand to an upstream stage of higher biochemical oxygen demand. This type of flow of the mixed liquor has been referred to as being "flow restricted", and is achieved by providing orifices of relatively small size in the partitions separating the stages such that the liquid flow is unidirectional from stage to stage.
On the other hand, previous systems are known such as, for example, that disclosed in U.S. Pat. No. 3,724,667 wherein the mixed liquor is not physically into stages, but rather, is completely mixed throughout the aeration basin so as to have a substantially uniform biochemical oxygen deman throughout the aeration basin. This latter type of system, referred to as a CMAS system, has the advantage that it will readily accommodate wide variations in the volume and strength of the influent, as well as, other types of shock loadings.
The present invention combines the advantages of both types of systems and is capable of achieving substantially the same advantages as the partitioned type of system without the need for such partitions extending vertically throughout the mixed liquor. Instead, it has been found that by selecting the initial design and/or controlling certain process parameters, the mixed liquor may be treated in a manner such that the amount of intermixing between non-partitioned zones of the basin may be controlled so as to achieve either the effect of a large number of physically separate liquid stages; i.e., "effective liquid staging," or to approach the conditions of a single zone, CMAS type system.
It is therefore a principal object of the present invention to provide an oxygenated waste treatment system wherein the amount of inter-zone mixing may be controlled so as to permit a more flexible operation which may be varied from conditions approaching the completely mixed mode of operation, to conditions approaching those of the physically partitioned, liquid stage mode of operation.
It is a further object of the present invention to provide an oxygenated waste treatment system having non-partitioned liquid zones while, at the same time, having a fixed or variable design capability of achieving a high degree of effective liquid staging.